Geology, of the High Climb Pegmatite, Custer County, South Dakota

Abstract

The High Climb pegmatite, Custer County, S. Dak., belongs to the series of pegmatitic and granitic rocks that characterize the Harney Peak region of the southern Black Hills. It intrudes pre-Cambrian metamorphic rocks consisting chiefly of quartz-mica schist. The country rock has been altered to a tourmaline-rich schist along part of the pegmatite contact. The structure of the pegmatite, in general is concordant with the westward-dipping schistosity of the country rock, but locally the pegmatite is crosscutting. The main part of the pegmatite is an irregularly shaped pipe that plunges 45° N. 40° W., parallel to the average plunge of rolls in the foot wall. A small northern extension of the pegmatite has a lenticular shape and crosscuts the schist at a low angle. Rolls in this part of the pegmatite have an average plunge of 28° N. 25° W. One large, north-trending crestal roll divides the outcrop of the northern segment of the pegmatite into two parts. The pegmatite has a well- defined internal structure consisting of five zones. A fine-grained wall zone, consisting of albite-quartz pegmatite, and a medium-grained first intermediate zone, consisting of albite-quartz-muscovite pegmatite, form incomplete concentric shells. Perthite-quartz-albite pegmatite (second intermediate zone) forms a hood-shaped unit between the outer units and the third intermediate zone along the crest and hanging-wall of the pegmatite. A concentric shell of quartz-cleavelandite pegmatite (third intermediate zone) surrounds a lenticular core of quartz pegmatite that contains altered spodumene, in addition -to the five zones, a fracture-filling unit of quartz-perthite-muscovite-albite-pegmatite and a possible sixth zone (or replacement unit?) of very fine-grained muscovite pegmatite were recognized. The essential minerals of the pegmatite include microcline-perthite, quartz, albite, and muscovite. Accessory minerals include tourmaline, beryl, amblygonite (variety, montebrasite), apatite, columbite-tantalite, loellingite, altered spodumene, numerous unidentified dark-colored phosphate minerals, a manganese-bearing carbonate, garnet, and chalcopyrite. An explanation of the origin of the pegmatite requires fractional crystallization and incomplete reaction in a restricted system. The concentric zonal structure and the general increase in grain size from the wall zone to the core suggest that the pegmatite units crystallized in order from the walls inward, without any true replacement stages. Industrial minerals that have been produced at the High Climb pegmatite are beryl, amblygonite, potash feldspar, columbite-tantalite, scrap mica, and sheet mica.